Chemical pulping (known as Kraft pulping) of wood chips for paper production involves alkaline sulphate cooking of the wood chips to remove 90-98% of the lignin present in the wood. The remaining 2-10% lignin imparts a dark brown colour to the pulp which tends to darken in UV light or by oxidation. To obtain a white pulp, the lignin present in the pulp must therefore be removed by a variety of bleaching procedures, most of which involve treatment with chlorine or chlorine dioxide, ozone, oxygen or hydrogen peroxide.
Due to an increasing concern about the environmental impact of the chemicals generated in the bleaching process, enzymatic treatment of the pulp has been proposed with a view to removing lignin from paper pulp while reducing the amount of bleaching chemicals needed in the process, vide e.g. "The third International Conference on Biotechnology in the Pulp and Paper industry", Stockholm, 16-19 Jun., 1986, pp. 67-69.
The enzymatic treatment of paper pulp hitherto described has mostly been carried out at an acid pH with hemicellulases with an acid pH optimum, vide e.g. "4th International symposium of Wood and Pulping Chemistry", Paris, 22-30 Apr., 1987, Vol. 1, pp. 151-154, or with a fungal preparation from Trichoderma, cf. M. G. Paice and L. Jurasek, J. Wood Chem. Technol. 4, 1989, pp. 187-198; or D. J. Senior et al., Biotechnol. Lett. 10, 1988, pp. 907-912, requiring a pH adjustment of the wood pulp to below a pH of 6.
Xylanase compositions are also used in the pulp and paper industry in the pulp bleaching process to enhance the brightness of bleached pulps, decrease the amount of chemicals used for bleaching as well as in the bleaching of recycled paper, cf. K. E. L. Eriksson, Wood Science and Technology 24, 1990, pp. 79-101, M. G. Paice et al., Biotechnol. and Bioeng. 32, 1988, pp. 235-239, J. C. Pommier et al., Tappi Journal, 1989, pp. 187-191. The use of an alkaline xylanase for pulp treatment is described in WO 91/02839.
In wood, lignin is linked to xylan. Xylanase is capable of catalysing the hydrolysis of the xylan so that an increased release of lignin occurs during bleaching. Another enzyme, cellobiose oxidase, has been found to be important for lignin degradation in that it reduces phenoxy radicals and guinones formed by the action of phenol oxidases on degradation products from lignin, thereby oxidising cellobiose and higher cellodextrins to the corresponding lactones. A cellobiose oxidase from Phanerochaete chrysosporium has been described in A. R. Ayers et al., Eur. J. Biochem. 90, 1978, pp. 171-181, and further characterized by F. F. Morpeth, Biochem. J. 228, 1985, pp. 557-564. This cellobiose oxidase was found to have a pH optimum at pH 5. Cellobiose oxidases have also been found in brown rot fungus Coniophora puteana (D. R. Schmidhalter and G. Canevascini, Appl. Microbiol. Biotechnol. 37, 1992, pp. 431-436) and soft rot fungi such as Monilia sp. (R. F. H. Dekker, J. Gen. Microbiol. 120, 1980, pp. 309-316), Chaetomium cellulolyticum (P. Fahnrich and K. Irrgang, Biotechnol. Lett. 4(12), 1982, pp. 775-780), Myceliophthora (Sporotrichum) thermophile (M.-R. Coudray et al., Biochem. J. 203, 1982, pp. 277-284) and Sclerotium rolfsii (J. C. Sadana and R. V. Patil, J. Gen. Microbiol. 131, 1985, pp. 1917-1923). It is believed that these cellobiose oxidases participate in cellulose degradation.